This invention relates to a method for manufacture of optical torque transducers. Such torque transducers are used for measuring the magnitude of torque in shafts, in particular rotating shafts such as found in electric power steering systems in vehicle applications.
Electric power steering systems conventionally incorporate an input shaft element, connected via a Hooke""s joint arrangement and intermediate shaft to the steering wheel. The input shaft therefore needs to rotate through an angle typically one to two revolutions either side of the on-centre steering position. It is a requirement of the electric power steering system to accurately measure the continuously varying torque in this rotating shaft. Conventionally torque applied to the shaft causes it to angularly deflect, such deflection causing one part of the shaft to angularly displace with respect to another part, and this displacement may be sensed to provide a measurement of this torque.
The sensing means needs to allow for rotation of the shaft, preferably employing non-contact signal transmission means for reliability or simplicity. Said sensing means includes aperture based and reflective optical devices and magnetic devices such as magnetostrictive or variable reluctance couplings.
To improve the accuracy of such sensing means the torque transducer may incorporate a shaft assembly which comprises two grating elements attached to or integral with two torque input members joined by a torsionally compliant coupling. When torque is applied between the two torque input members the torsionally compliant coupling deflects causing an increased relative angular displacement of the two grating elements that allows the use of less sensitive sensing means.
This method relates to the manufacture of torque transducers that use reflective or transmissive optical sensing means, which comprise grating patterns consisting of alternating regions of high and low reflectivity or transmissibility (respectively) for a given incident electromagnetic radiation (EMR). High and low reflectivity includes variations in direct, specular reflection and variations in diffuse reflection. The grating patterns are illuminated by a source of EMR, typically UV, visible or IR light, which are interrogated by one or more arrays of detectors sensitive to the EMR. Arrays include Charge Couple Devices (CCD), Very Large Scale Integration (VLSI) vision chips and one or two-dimensional photodetector arrays. The output of the one or more arrays interrogating the patterns can be processed to produce a measure of the torque applied to the shaft.
The regions of high and low reflectivity or transmissibility can be arranged axially or radially about the axis of rotation of the shaft, and are of such a nature that allows a continuous output of the arrays regardless of the angular position of the shaft, as the array dimensions may not allow the complete circumference or radial face to be viewed by the arrays at any instant in time.
The prior art which is most closely related to that of the present invention is described in U.S. Pat. No. 4,406,939 (Golker) entitled xe2x80x9cMethod for the Manufacture of a Code Disk for Optical Incremental Shaft Encoders and Absolute Shaft Encodersxe2x80x9d which shows the use of laser techniques for manufacturing of shaft encoder patterns.
The essence of the present method resides in the application of radiation, and particularly optical laser radiation, patterning techniques simultaneously to two mutually adjacent grating elements of a shaft assembly. The shaft assembly is assembled prior to irradiation, and comprises the two grating elements attached to or integral with two torque input members that are connected by a torsionally compliant coupling. The simultaneous irradiation of the two grating elements of the shaft assembly provides very accurate control of the relative disposition of the two grating patterns thus generated for the zero torque alignment condition. Further, since the grating patterns are mutually adjacent, a single radiation source can be used for both patterns with further improvement of pattern disposition. This contrasts to the use of optical patterning methods as disclosed in the aforementioned prior art which, if employed, would require the independent xe2x80x9cprepatterningxe2x80x9d of two grating elements (referred to as xe2x80x9ccode disksxe2x80x9d) prior to assembly of the shaft, with subsequent introduction of inaccuracies in the relative disposition of the two grating patterns during the subsequent assembly operation.
Radiation in the present specification includes optical electromagnetic radiation with wavelengths in vacuum between 40 nm and 1 mm, as defined by The International Society for Optical Engineering.
The advantages of a method according to the present invention to construct a xe2x80x9creflective gratingxe2x80x9d torque transducer (as disclosed in co-pending International Patent Application No. PCT/AU98/00645) or a xe2x80x9ctransmissive gratingxe2x80x9d torque transducer (as disclosed in co-pending Australian Provisional Patent Application No. PP0984 filed Dec. 17, 1997 entitled xe2x80x9cTransmission Path Torque Transducerxe2x80x9d) accrue from the generation of the patterns after assembly of the shaft. This means that the relative dispositions of the two patterns are determined with great accuracy, which is not disturbed by subsequent assembly operations as would be the case if the patterns were generated prior to assembly of the shaft.
Firstly, calibration of the finished transducer is not required thus saving cost and time in manufacture.
Secondly, the shafts can be interchanged without recalibration or accurate assembly methods, and the arrays are likewise interchangeable without recalibration or accurate assembly methods.
Thirdly, the transducer can be disassembled and reassembled if required for maintenance or repair purposes without the need for special tools or accurate assembly methods.
Fourthly, the xe2x80x9clocked inxe2x80x9d nature of the relative pattern dispositions means that adjustment features such as screws and stops are not required, which therefore eliminates the possibility that the transducer can be inadvertently miscalibrated by the user or untrained repair personnel.
Fifthly, the use of irradiation allows patterns to be generated on the grating elements with great accuracy and speed.
Finally, the use of irradiation allows complex and angularly non-repetitive patterns, such as barcodes, to be flexibly generated, for example by manipulation of a laser. This allows the torque transducer shaft to be of simpler construction as xe2x80x9caliasingxe2x80x9d problems can be eliminated as the individual marks (or groups of regions of high and low reflectivity or transmissibility) have a unique, coded identity. Aliasing is defined in this application as the situation where sufficient angular deflection occurs between the two grating elements that the disposition of the respective patterns is identical to their disposition for a lesser magnitude of angular deflection, thereby yielding indeterminancy and potentially a false measurement of torque in the shaft. Barcodes provide an absolute indication of which mark is being viewed by the array hence eliminate indeterminacy caused by aliasing. Marks in the form of barcoded patterns also allow easy determination of absolute shaft angular position, providing additional functionality of the transducer, or elimination of the need for external angle encoders.
The present invention consists in a method of manufacturing grating patterns on the surfaces of grating elements of a torque transducer, the torque transducer comprising a shaft with a longitudinal axis, the shaft comprising first and second substantially rigid torque input members which are connected by a torsionally compliant coupling, the coupling thereby enabling angular deflection of the first torque input member relative to the second torque input member as a function of the magnitude of the torque in the shaft, a first grating element attached to or integral with the first torque input member and a second grating element attached to or integral with the second torque input member, the surfaces of the two grating elements being adjacent and able to relatively displace as a function of torque in the shaft, the method comprising the steps of:
a first step of forming an assembly by rotationally and axially fixing the first and second torque input members and respective grating elements to the torsionally compliant coupling; a second step of mounting the assembly in a machine comprising a radiation source arranged to irradiate regions of the adjacent surfaces of the two grating elements whilst a predetermined relative angular displacement between the two torque input members with respect to their zero torque alignment condition is maintained; and a third step of simultaneously irradiating regions of the adjacent surfaces of the two grating elements whilst the predetermined angular displacement between the two torque input members is maintained; thereby generating grating patterns on the surfaces of the two grating elements of the assembly which are precisely mutually aligned for this zero torque alignment condition.
In a first embodiment it is preferred that the machine provides mounting for rotation of the assembly about the longitudinal axis of the shaft and the third step further comprises rotating the assembly and successively irradiating regions of the adjacent surfaces at predetermined angular rotational positions of the assembly.
It is preferred that the assembly rotates through substantially one revolution during the successive rotations.
It is preferred that the assembly is rotationally stationary during at least one of the successive irradiations.
Alternatively it is preferred that the assembly is rotating during at least one of the successive irradiations.
In a second embodiment it is preferred that the assembly is mounted stationary in the machine during the third step.
It is preferred that the surface of at least one of the first or second grating elements is substantially cylindrical, with its central axis collinear with the longitudinal axis of the shaft.
Alternatively it is preferred that the surface of at least one of the first or second grating elements is substantially planar and radially disposed with respect to the longitudinal axis of the shaft.
Still alternatively, it is preferred that the surface of at least one of the first or second grating elements is substantially conical, with its central axis collinear with the longitudinal axis of the shaft.
Still alternatively, it is preferred that the surface of at least one of the first or second grating elements is substantially axis-symmetric, with its central axis collinear with the longitudinal axis of the shaft.
It is preferred that each grating pattern comprises alternating regions of high and low reflectivity or transmissibility for a given incident electromagnetic radiation.
It is preferred that a radiation-opaque mask is interposed between the radiation source and the surfaces of the grating elements and the geometry of the mask determines the shape and disposition of the patterns.
Alternatively the radiation source comprises a laser that irradiates the surfaces of the grating elements and the shape and disposition of the patterns are determined by controlling the laser beam focus and/or impingement position on the grating elements.
Alternatively the radiation source comprises a laser that irradiates the surfaces of the grating elements and the shape and disposition of the patterns are determined by controlling the relative positions of the grating elements and the laser.
It is preferred that the predetermined relative angular displacement maintained between the two torque input members is substantially zero.
It is preferred that the grating pattern on at least one of the grating elements is arranged in the form of a bar code or a succession of bar codes.
It is preferred that the irradiation removes material or changes the physical or chemical properties of regions of the adjacent surfaces of the grating elements, thereby increasing or decreasing the reflectivity or transmissibility of these regions.
Alternatively a layer of polymer such as polyimide is applied to the adjacent surfaces of the grating elements prior to irradiation, and the irradiation removes this layer from regions of the grating elements, thereby exposing the original surface of the grating elements, this being of increased or decreased reflectivity or transmissibility compared to the surface of the polymer layer.
Alternatively a layer of inorganic material such as alumina ceramic is applied to the adjacent surfaces of the grating elements prior to irradiation, and the irradiation removes this layer from regions of the grating elements, thereby exposing the original surface of the grating elements, this being of increased or decreased reflectivity or transmissibility compared to the surface of the inorganic layer.
Alternatively a layer of metallic material such as electroplated copper is applied to the adjacent surfaces of the grating elements prior to irradiation, and the irradiation removes this layer from regions of the grating elements, thereby exposing the original surface of the grating elements, this being of increased or decreased reflectivity or transmissibility compared to the surface of the metallic layer.
Alternatively the surfaces comprise a proportion of titanium dioxide and the change in chemical properties is effected by changing the titanium dioxide to titanium oxide.
Alternatively the surfaces comprise a proportion of aluminium oxide, and the change in chemical properties is effected by reduction of aluminium oxide to aluminium.
Alternatively the irradiation cures regions of a layer of uncured polymer applied to the adjacent surfaces of the grating elements prior to irradiation, the surfaces of these regions being of increased or decreased reflectivity or transmissibility compared to the original surface of the grating elements which are subsequently exposed by removal of the remaining uncured polymer after irradiation.
It is preferred that the assembly is subjected to turning, grinding, roller burnishing, honing or other processes to improve the relative cylindricity or concentricity of the surface of the at least one substantially cylindrical grating element or, alternatively, to improve the relative flatness or planarity of the surface of the at least one substantially planar and radially disposed grating element or, still alternatively, to improve the relative conicity or concentricity of the surface of the at least one substantially conical grating element, before the grating pattern is generated.